1. Field of the Invention
The present invention is directed to downhole tools. More specifically, the present invention is directed to a bi-center drilling bit adapted to fit within and drill through a casing shoe without damage to the surrounding casing.
2. Background
Bi-center bits are adapted for insertion down a wellbore having a given diameter where, once in position, the rotation of the bi-center bit creates a borehole having a selectedly greater diameter than the borehole.
In conventional bi-center bits, the bit is designed to rotate about a rototial axis which generally corresponds to the rotational axis defined by the drill string. Such conventional designs are further provided with cutting elements positioned about the face of the tool to reveal a low backrake angle so as to provide maximum cutting efficiency.
Disadvantages of such conventional bi-center bits lie in their inability to operate as a cutting tool within their pass-through diameter while still retaining the ability to function as a traditional bi-center bit. In such a fashion, a conventional bi-center bit which is operated within casing of its pass-through diameter will substantially damage, if not destroy the casing.
The present invention addresses the above and other disadvantages of prior bi-center drilling bits by allowing selective modification of the use of the tool within the borehole.
In one embodiment, the present invention includes a drill bit body which defines a pilot section, a reamer section and a geometric axis. The pilot section defines a typical cutting surface about which is disposed a plurality of cutting elements. These elements are situated about the cutting face to generally define a second rototional axis separate from the rotational axis defined by the drill string as a whole. This second or pass-through axis is formed by the rotation of the bit about the pass-through diameter.
In one embodiment, the pilot section may define a smaller diametrical cross-section so as to further prevent the possibility of damage to the borehole and/or casing when the bit is rotated about the pass-through axis. To further accomplish this goal, a gauge pad may also be situated on the drill bit body opposite the reamer. In yet other embodiments, cutters emphasizing a high back rake angle are employed on the peripheral cutting blades of the tool.
The present invention presents a number of advantages over prior art bi-center bits. One such advantage is the ability of the bi-center bit to operate within a borehole or casing approximating its pass-through diameter without damaging the borehole or casing. In the instance of use in casing, the casing shoe may thus be drilled through.
A second advantage is the ability of the same tool to be used as a conventional bi-center bit to create a borehole having a diameter greater than its pass-through diameter. In such a fashion, considerable cost savings may be observed since only one tool need be used where this tool need not be retrieved to the surface to modify its character of use.